1. Field of the Invention
The present invention relates to liquid crystal display apparatus, and to methods of operating a display or spatial light modulator, in which the instantaneous intensity distribution afforded by the display or modulator is binary in nature but which is altered in a manner such that the time averaged distribution effectively has, or appears to have, multiple intensity levels. For display purposes, this means that the alteration must be sufficiently fast for averaging to occur at the eye, preferably avoiding any flicker. This requirement may or may not apply for other purposes.
The methods of the invention can be used in conjunction with any spatial light modulator capable of producing a binary image, including those comprising an array of individually addressable cells or pixels, and those where the binary image is produced by scanning of a modulated light beam, for example. The term “binary spatial light modulator” used herein is intended to encompass all such devices, whether they are used for display or other purposes, for example information recordal, and variable components (for example lenses, filters and diffraction gratings) in optical systems. The term is intended to cover passive modulators where an existing light beam is affected by the modulator, and also those which act as light sources, for example arrays of light emitters, and electroluminescent devices. The apparatus of the invention includes a passive binary spatial light modulator in the form of an array of liquid crystal pixels.
2. Discussion of Prior Art
The term “image” as used herein is used to denote any spatially varied light distribution, normally, but not necessarily, of light intensity, and its production or resulting distribution will be referred to by the term “display”.
Furthermore, although the term “grey scale” is used herein as denoting a multi-level distribution, it should be made clear that the term is used in relation to any colour, including white. In addition, although the methods, arrays, backplanes, circuitry etc. of the invention and its embodiment are described in relation to a single colour (monochrome images), including white, it is envisaged that variable colour images or displays etc. will be produced in manners known per se, such as by spatially subdividing a single an-ay into different colour pixels, superimposing displays from differently coloured monochrome arrays for example by projection, or temporal multiplexing, for example sequential projection of red green and blue images.
Temporally varying binary modulation to achieve a multiple intensity effect is known, and can be effected by the use of multiple bit planes. In such a scheme, an array of digitised values, of amplitudes corresponding to the grey scale values allocated to the pixels of the array, is decomposed into a multiplicity of bit planes. This multiple bit plane technique may be used with any binary spatial light modulator as defined above.
It is possible to decompose a n-level grey scale image into a plurality of binary image planes of equal duration, with a corresponding plurality of bit planes of equal duration. However, in a preferred form, known as a weighted bit plane technique, the durations of the bit planes are weighted, each bit plane being representative of one level (exponent) of the digitisation. This reduces the number of bit planes required to synthesise an image, and reduces addressing requirements somewhat. Examples of this technique are to be found in European Patent Application No. 0 774 745 (Matsushita Electronics), and JP 05 127612 (Nippon Hoso Kyokai).
Although in certain cases, it would be possible to use digital bases other than 2, this complicates matters insofar as each bit plane is not binary and thus is not so easily stored. Furthermore, each location of such a bit plane would then have more than one non-zero value, and the variation in non-zero values across the bit plane would need to be taken into account for the durations of operation of each pixel (possibly by further decomposing the non-binary plane to two or more binary planes). The discussion below will be limited to binary weighting, but the principles set out in such a context are believed to be sufficient to enable the skilled person to extrapolate to other exponential bases if required or desired.
Where the digitisation is binary, so that each bit plane is an array of digital 1s and 0s, it is then only necessary to display each bit plane for a total period proportional to its binary weighting to provide a time averaged image equivalent to the digitised grey scale image.
Where possible, it is convenient to display each binary bit plane once for the total duration necessary to contribute to the grey scale image, but it is also possible to display one or more of the bit planes a plurality of times, not necessarily sequentially, provided that the total time spent in displaying each bit plane, relative to the total time spent in displaying all the bit planes, is proportional to its binary weighting.
Recently there has been developed a novel spatial light modulator in the form of a smectic liquid crystal layer disposed between an active semiconductor backplane and a common front electrode. It was developed in response to a requirement for a fast and, if possible, inexpensive, spatial light modulator comprising a relatively large number of pixels (320×240 up to 640×480) with potential application not only as a display device, but also for other forms of optical processing such as correlation and holographic switching. Depending on the manner in which it is driven, and the value of the applied voltage, the modulator may be driven at a line rate of at least 10 MHz and a frame rate of up to 15 to 20 kHz, requiring a data input of around 1 to 1.5 Gpixel per second Typically, while the pixel address time is around 100 nanoseconds, the pixel will actually take around 1 to 5 microseconds to switch between optical states; and while overall frame writing time is of the order of 24 microseconds, the frame to frame writing period is around 80 microseconds.
This spatial light modulator can be driven according to single pass schemes, in which the front electrode is placed at a potential of V/2 relative to the backplane pixels, which are switched to zero volts or V volts.
Alternatively it can be driven according to double pass schemes in which in one pass the front electrode is placed at zero volts and selected pixels are turned ON by switching pixel elements of the backplane array to V volts, and in the other pass the front electrode is placed at V volts and selected pixels are turned OFF by switching elements of the array to zero volts. For pixels which are not in the process of being switched the elements of the backplane follow the voltage of the front electrode. To maintain the same potential difference therebetween, the voltage at all backplane pixel elements of the array is simultaneously switched as the voltage on the front electrode is changed between zero and V volts.
Our copending International Patent Applications (PCT/GB99/04285, ref: P20957WO, priority GB9827952.4; PCT/GB99/04286 and PCT/GB99/04276,. refs: P20958WO and P20958WO1, both priority GB9827965.6; PCT/GB99/04282, ref: P20959WO, priority GB9827900.3; PCT/GB99/04279, ref: P20960WO, priority GB9827901.1; PCT/GB99104274, ref: P20961WO, priority GB9827964.9; PCT/GB99/04275, ref: P20962WO, priority GB9827945.8; and PCT/GB99/04277, ref: P20963WO1, priority GB 9827944.1) relate to other inventive aspects associated with this spatial light modulator, including the single and double pass schemes referred to in the preceding paragraph
The aforesaid spatial light modulator is ideally suited to the use of the bit plane technique mentioned above. However, the present invention is not limited to liquid crystal modulators, but can be applied to any spatial light modulator as referred to above.
One problem which arises, particularly when operating liquid crystal display and modulators, is that of maintaining a dc balance at individual pixels. Initially, liquid crystal light modulators were in the form of a single cell comprising a layer of liquid crystal material sandwiched between opposed electrode bearing plates, at least one of the plates being transparent. Such cells were slow to operate and tended to have a short life due to degradation of the liquid crystal material. Quite early on it was recognised that the application of an average finite dc voltage to the liquid crystal cell was not beneficial, and at least in some cases produced degradation by electrolysis of the liquid crystal material itself, and schemes were evolved to render the average dc voltage to zero (dc balance).
It is now appreciated that other effects are also at work when a dc voltage is applied. When driving liquid crystal electro-optic devices for any length of time, a phenomenon known as image sticking may occur. Although the precise cause of this effect is unknown, there are theories that ions are trapped or a space charge is induced within the material in response to an overall dc field, and this results in a residual field even when the external dc field is removed.
It is evidently desirable that the time averaged voltage (that is, the average over the time that the voltage is actually being applied from an external source to the liquid crystal) applied to a liquid crystal material is zero, whether to avoid degradation or to avoid image sticking.